Cascaded Raman laser

ABSTRACT

A cascaded Raman laser ( 10 ) has a pump radiation source ( 12 ) emitting at a pump wavelength λ p , an input section ( 14 ) and an output section ( 16 ) made of an optical medium. Each section ( 14, 16 ) comprises wavelength selectors ( 141, 142, . . . , 145  and  161, 162, . . . , 165 ) for wavelengths λ 1 , λ 2 , . . . , λ n−k , where n≧3, λ p &lt;λ 1 &lt;λ 2 &lt; . . . &lt;λ n−1 &lt;λ n  and λ n−k+1 , λ n−k+2 , . . . , λ n  being k≧1 emitting wavelengths of the laser ( 10 ). The laser further comprises an intracavity section ( 18 ) that is made of a non-linear optical medium, has a zero-dispersion wavelength λ 0  and is disposed between the input ( 14 ) and the output ( 16 ) section. The wavelengths λ 1 , λ 2 , . . . , λ n−k  of the wavelength selectors ( 141, 142, . . . , 145  and  161, 162, . . . , 165 ) and the zero-dispersion wavelength λ 0  of the intracavity section ( 18 ) are chosen such that energy is transferred between different wavelengths by multi-wave mixing.

BACKGROUND OF THE INVENTION

[0001] The invention is based on a priority application EP 02360221.2which is hereby incorporated by reference.

[0002] The invention relates to a cascaded Raman laser comprising a pumpradiation source emitting at a pump wavelength λ_(p), an input sectionand an output section made of an optical medium, each section comprisingwavelength selectors for wavelengths λ₁, λ₂, . . . , λ_(n−k), where n≧3,λ_(p)<λ₁<λ₂< . . . <λ_(n−1)<λ_(n) and λ_(n−k+1), λ_(n−k+2), . . . ,λ_(n) being k≧1 emitting wavelengths of the laser, and an intracavitysection that is made of a non-linear optical medium, has azero-dispersion wavelength λ₀ and is disposed between the input and theoutput section.

[0003] Such lasers are generally known in the art, for example from U.S.Pat. No. 5,323,404.

[0004] Raman lasers typically comprise a pump source, usually acontinuos wave (CW) laser, and a length of an non-linear optical medium,for example an optical fiber. Two reflectors having the same peakreflectivity are spaced apart on the optical fiber so as to form anoptical laser cavity. The underlying physical principle of such lasersis the effect of spontaneous Raman scattering. This is a non-linearoptical process that only occurs at high optical intensities andinvolves coupling of light propagating through the non-linear medium tovibrational modes of the medium, and re-radiation of the light at adifferent wavelength. Re-radiated light upshifted in wavelength iscommonly referred to as a Stokes line, whereas light downshifted inwavelength is referred to as an Anti-Stokes line. Raman lasers aretypically used an a configuration in which pump light is upshifted inwavelength. When a silica fiber is used as the non-linear medium, thestrongest Raman scattering (maximum Raman gain) occurs at a frequencyshift of about 13.2 THz, corresponding to a wavelength shift of about50-100 nm for pump wavelengths between about 1 and 1.5 μm.

[0005] In Raman lasers that are configured as ring lasers, thenon-linear optical fiber is closed by a coupler or an optical circulatorso that a fiber loop is obtained. The reflectors are then usuallyreplaced by optical filters, for example Fabry-Perot-Filters, having aspecified passband center wavelength. The generic term “wavelengthselectors” will henceforth be used for designating reflectors, filtersor other means that are used to define optical resonators in Ramanlasers.

[0006] A “cascaded” Raman laser is a Raman laser that has, in additionto an optical cavity for radiation of an emission wavelength λ_(n), atleast one further optical cavity for radiation of wavelengthλ_(n−1)<λ_(n), where n≧2. In such cascaded Raman lasers radiationundergoes more than one Stokes transitions so that it is subsequentlyupshifted in wavelength. If radiation with more than one wavelength iscoupled out of the laser, such a laser is referred to as amulti-wavelength Raman laser.

[0007] Raman lasers are often used as pump lasers for Raman amplifiersat 1310 or 1550 nm, or as 1480 nm pump lasers for remotely pumped erbiumfiber amplifiers in repeaterless optical fiber communication systems.Uses for other purposes at other wavelengths are possible andcontemplated.

[0008] One of the key properties of cascaded Raman lasers is theconversion efficiency which is defined as the ratio between output powerof the laser and optical pump power at the input side. Another keyproperty is the threshold pump power that has to be exceeded forgenerating a substantial optical output power, i.e. an output power ofat least some mW. With pump powers below the threshold, there is only aninsignificant optical output power in the order of several μW.

[0009] A Raman laser having a low pump threshold is desirable in manyrespects. For example, it should allow to generate a low but stableoutput power. Although in many applications high output powers are anessential feature, there are other applications which require such lowand stable output powers.

SUMMARY OF THE INVENTION

[0010] An example for such an application is a Raman laser for secondorder pumping. In a typical configuration of such a second order pumplaser, two low-power pump sources are boosted, pursuant to the firstorder pumping principle as explained above, by a single high-powersource. The two low-power sources may then, for example, amplify atransmission signal. Such a configuration is considerably less expensivethan current long-haul transmission systems which require the use of twohigh-power pump sources.

[0011] In principle, such a configuration would require low-pqwer pumpsources emitting only a few mW. Currently, however, pump lasers withsuch low but stable output powers are not available for the requiredwavelength range.

[0012] From a paper by J. -C. Bouteiller et al. entitled “Dual-orderRaman pump providing improved noise figure and large gain bandwith”,FB3-1, OFC 2002 Postdeadline Paper, it is known to combine thehigh-power and the low-power sources in a single cascaded Raman laserdevice. The stable low-power output is achieved by attenuating anoriginally stable and high-power laser line in a narrow long periodgrating.

[0013] From US-A1-2002/0015219 a non-linear fiber amplifier is knownthat is particularly suited for the low-loss window at approximately1430-1530 nm. This broadband non-linear polarization amplifier combinescascaded Raman amplification with parametric amplification or four-wavemixing. One of the intermediate cascade Raman order wavelengths shouldlie in close proximity to the zero-dispersion wavelength λ₀ of theamplifying fiber. For this intermediate Raman order, spectral broadeningwill occur due to phase-match with four-wave mixing or phase-matchedparametric amplification. In further cascaded Raman orders, the gainspectrum continues to broaden due to the convolution of the gainspectrum with a spectrum from the previous Raman order. This document,however, does not relate to the issue of cascaded Raman lasers with lowpump power threshold.

[0014] It is, therefore, an object of the present invention to provide acascaded Raman laser as mentioned at the outset that has a low pumppower threshold.

[0015] This object is achieved, with a laser as mentioned at the outset,in that the wavelengths λ₁, λ₂, . . . , λ_(n−k) of the wavelengthselectors and the zero-dispersion wavelength λ₀ of the intracavitysection are chosen such that energy is transferred between radiation ofdifferent wavelengths by multi-wave mixing.

[0016] Since Raman scattering is now assisted by multi-wave mixing,energy is transferred from radiation with shorter wavelengths toradiation with longer wavelengths more efficiently. This boosting ofStokes transitions by multi-wave mixing allows to achieve a stablelow-power output. The new cascaded Raman laser is therefore particularlysuited as a low-power pump source in configurations with second orderpumping as explained above, but of course not restricted to such use.

[0017] The more efficient energy transfer between Stokes lines has alsothe advantage that the last Stokes line(s) corresponding to the emittingwavelength(s) of the laser appear(s), in a spatial sense, sooner in thepropagation direction of the light. Thus, it is possible to reduce thelength of the intracavity section, for example a Raman active fiber.

[0018] The term multi-wave mixing is used herein as a generic term forthe phenomena of four-wave mixing and degenerated four-wave mixing, thelatter being often referred to as three-wave mixing.

[0019] Each pair of wavelength selectors for a specific wavelength formsan optical cavity for radiation of a wavelength equal to thiswavelength. The wavelength selectors may be realized as reflectors, forexample Bragg gratings, having a specific center wavelength which isdefined as the peak wavelength of the reflectivity band of thereflector.

[0020] The input and the output sections are considered to be thosesections of the Raman laser that contain the wavelength selectors. Theintracavity section of the laser is a central section without selectorsand is disposed between the input section and the output section. Thereis no constraint with respect to the optical mediums that form theinput, the output and the intracavity section, as long as only theintracavity section is made of materials showing non-linear effects,particularly spontaneous Raman scattering, when exposed to high opticalintensities. The sections may be formed as different optical mediums andmade of different materials, but it is also possible that the input, theoutput and the intracavity section is formed as a single continueswaveguiding structure that is divided into different sections onlyconceptionally.

[0021] The desired effect of energy transfer by multi-wave mixingrequires that the zero-dispersion wavelength λ₀ is chosen such thatlinear phase matching occurs. The applicable phase matching conditionwill depend on the Stokes transitions that shall be assisted bymulti-wave mixing. However, this does not mean that the applicable phasematching condition will have to be fulfilled exactly. Deviations of thezero-dispersion wavelength λ₀ from the ideal value of +/−10 nm willstill allow multi-wave mixing to become sufficiently large.

[0022] Multi-wave mixing does not necessarily has to involve any of thewavelengths that are to be coupled out of the laser. However, it isparticularly preferred if at least one of the emitting wavelengths ofthe laser is involved in multi-wave mixing. If only one wavelength isemitted by the laser, this means that additional energy is transferredto the emitted radiation by multi-wave mixing.

[0023] The wavelengths λ_(1, λ2), . . . , λ_(n−k) of the wavelengthsselectors may be chosen so that energy transfer by multi-wave mixinginvolves at least three adjacent wavelengths. This is particularlyadvantageous if only radiation of one wavelength shall be coupled out ofthe laser.

[0024] In order to achieve four-wave mixing in this case, the centerwavelength of the reflectors have to be chosen so that

1/λ_(i)=1/λ_(i−1)+1/λ_(i−2)−1/λ_(i−3),

[0025] where i=3, 4, . . . , n, and that the zero-dispersion wavelengthλ₀ of the intracavity section (18) substantially equals(λ_(i−1)+λ_(i−2))/2.

[0026] In order to achieve three-wave mixing in this case, thewavelengths λ₁, λ₂, . . . , λ_(n−k) of the wavelengths selectors have tobe chosen so that

1/λ_(i)=2/λ_(i−1)−1/λ_(i−2),

[0027] where i=3, 4, . . . , n, and that the zero-dispersion wavelengthλ₀ of the intracavity section (18) substantially equals λ_(i−1).

[0028] However, it is not required that multi-wave mixing involves onlyadjacent Stokes lines. Particularly for lasers emitting radiation withmore than one wavelength it is preferred if the wavelengths λ₁, λ₂, . .. , λ_(n−k) of the wavelengths selectors are chosen so that energytransfer by multi-wave mixing involves at least three non-adjacentwavelengths.

[0029] For example, if radiation of two different wavelengths shall becoupled out of the laser, four-wave mixing may be achieved if thewavelengths λ₁, λ₂, . . . , λ_(n−2) of the wavelengths selectors arechosen so that

1/λ_(i)=1/λ_(i−2)+1/λ_(i−3)−1/λ_(i−5)

[0030] and

1/λ_(i−1)=1/λ_(i−2)+1/λ_(i−3)−1/λ_(i−4),

[0031] where i=5, 6, . . . , n, and that the zero-dispersion wavelengthλ₀ of the intracavity section (18) substantially equals(λ_(i−2)+λ_(i−3))/2.

[0032] The condition for three-wave mixing is in this case that thewavelengths λ₁, λ₂, . . . , λ_(n−1) of the wavelengths selectors arechosen so that

1/λ_(i)=2/λ_(i−2)−1/λ_(i−4)

[0033] and

1/λ_(i−1)=2/λ_(i−2)−1/λ_(i−3),

[0034] where i=5, 6, . . . , n, and that the zero-dispersion wavelengthλ₀ of the intracavity section (18) substantially equals λ_(i−2).

[0035] It should be noted, however, that multi-wave mixing can beachieved in various other ways that differ from the exemplary conditionsgiven above.

[0036] Due to the assistance of multi-wave mixing it is not necessary toprovide optical cavities also for those emitted wavelengths that profitfrom multi-wave mixing in the sense that energy is transferred moreefficiently to radiation of those wavelengths.

[0037] However, in order to improve laser stability it is preferred thatfor each emitting wavelength an additional wavelength selector forwavelength λ_(n−k+1), λ_(n−k+2), . . . , λ_(n), respectively, isprovided in the input section and in the output section.

[0038] The components of the new cascaded Raman laser have so far beenreferred to in general terms. For example, no restrictions are made asto whether the laser is realized in a linear or in a ring laserconfiguration. The input and the output sections as well as theintracavity section may be formed by any known waveguiding structure,for example a planar optical waveguide.

[0039] It is, however, particularly preferred if the input and theoutput sections are optical fibers and the intracavity section is aRaman active optical fiber, as is known in the art as such.

[0040] The type of selector chosen will depend on the particularwaveguiding structure that is used for the new laser. For opticalfibers, in principle any reflecting means having a high reflectivity,for example a multilayer structure formed directly on a fiber end face,could be used. Particularly preferred reflectors for optical fibers arein-line Bragg gratings.

BRIEF DESCRIPTION OF THE DRAWINGS

[0041] The above and other advantages and features of the presentinvention will become apparent from the following description of apreferred embodiment given in conjunction with the accompanyingdrawings, in which:

[0042]FIG. 1 shows a schematic diagram of a cascaded Raman fiber laseraccording to the invention;

[0043]FIG. 2 is a schematic representation of the Stokes lines of thelaser shown in FIG. 1 in the wavelength domain, illustrating the energytransfer between adjacent lines;

[0044]FIG. 3a is a simplified energy level diagram illustrating theprocess of four-wave mixing in an optical fiber;

[0045]FIG. 3b is a schematic diagram illustrating four-wave mixing inthe wavelength domain;

[0046]FIG. 4a is a simplified energy level diagram illustrating theprocess of three-wave mixing in an optical fiber in a representationsimilar to that of FIG. 3a;

[0047]FIG. 4b is a schematic diagram illustrating three-wave mixing inthe wavelength domain in a representation similar to that of FIG. 3b;

[0048]FIG. 5 is a graph showing experimental results of the opticalpower present at different wavelengths for increasing pump power Plin ina conventional laser, illustrating the pump threshold for the excitationof different Stokes lines;

[0049]FIG. 6 is a similar graph as in FIG. 5, but for a laser accordingto the present invention;

[0050]FIG. 7 is a schematic representation of the Stokes lines in thefrequency domain of another embodiment of a laser according to theinvention, illustrating the energy transfer between adjacent pairs ofStokes lines with assisting four-wave mixing;

[0051]FIG. 8 is a schematic representation of the Stokes lines in thefrequency domain of a still further embodiment of a laser according tothe invention, illustrating the energy transfer between adjacent pairsof Stokes lines with assisting three-wave mixing.

[0052]FIG. 1 schematically depicts an exemplary embodiment of a cascadedRaman laser according to the invention being designated in its entiretyby 10. Laser 10 comprises a pump source 12 emitting radiation ofwavelength λ_(p). Neodymium or ytterbium fiber lasers or another Ramanlaser may be used as pump source 12. Furthermore, any other single-modelaser, both fiber and based on voluminous elements, including crystals,doped with metal ions, may be used as a pump source 12.

[0053] Laser 10 further comprises an input section 14, an output section16 and an intracavity section 18 that is disposed between input section14 and output section 16. Intracavity section 18 consists, in thisembodiment, of a phosphosilicate Raman active fiber 20 displaying astrong non-linear response to high-power optical intensities. Ramanactive fibers that are particularly designed for this purpose are knownin the art as such and will, therefore, not be described in more detail.

[0054] Input section 14 comprises an input fiber 21 which is a normallow loss optical fiber. However, input fiber 21 can, in principle, alsobe a Raman active fiber. In the embodiment shown, N=5 fiber Bragggratings 141, 142, . . . , 145 are formed on input fiber 21.

[0055] Output section 16 comprises an output fiber 23 that is of thesame kind as input fiber 23. However, output fiber 23 can, in principle,also be a Raman active fiber or be of a type distinct from input fiber21. In the output section 16 an equal number of N=5 optical fiber Bragggratings 161, 162, . . . , 165 are formed on output fiber 23.

[0056] The input group of Bragg gratings 141, 142, . . . , 145 and theoutput group of Bragg gratings 161, 162, . . . , 165 are used asreflectors having reflectivity bands with center wavelengths λ₁, λ₂, . .. , λ₅, respectively. Each pair of Bragg gratings having a matchingcenter wavelength λ_(i) forms an optical resonator for wavelength λ_(i)that comprises at least a part of the length of the Raman active fiber20 of the intracavity section 18.

[0057] The cavity length may, for example, be in the range of a fewhundred meters up to a few kilometers. Since the attenuation of Ramanactive fiber 20, input fiber 21 and output fiber 23 typically depends onthe wavelength, the optimum cavity length for given wavelengths will bedifferent for different wavelengths. Since in-line fiber Bragg gratingscan be essentially 100% transmissive at wavelengths outside of theirreflection band, a flexible placement of all Bragg gratings is possible.For instance, the optical cavities may be sequential or overlapping tovarious degrees. In the embodiment shown in FIG. 1, it is assumed thatλ₁<λ₂< . . . <λ₅. As can be seen in FIG. 1, the cavity lengths increasewith growing center wavelengths.

[0058] All of the above-mentioned Bragg gratings desirably have highreflectivity of preferably more than 98% at their center wavelength.Only Bragg grating 165 in output section 16 has a low reflectivity(typically in the range between 5% and 15%) for radiation of wavelengthλ₅. This is because wavelength λ₅ is the emitting wavelength of laser 10that shall, at least to a large degree, be coupled out of laser 10.Bragg grating 165 can also be replaced by a cleaved fiber end thatprovides sufficient reflectivity.

[0059] Laser 10 further comprises in its output section 16 an additionalunpaired high reflectivity Bragg grating 22 having a center wavelengthλ_(p). This unpaired Bragg grating 22 reflects pump light withwavelength λ_(p) and thus provides for at least dual passage of the pumpradiation along Raman active fiber 20 and, thus, more effective use ofpump radiation.

[0060] Reference numerals 24 and 26 designate a first and a secondwelding point at which Raman active fiber 20 of the intracavity section18 is connected to input fiber 21 and to output fiber 23, respectively.However, as already mentioned above, all Bragg gratings may be includeddirectly in Raman active fiber 20 so that no welding points arenecessary. Such a variant is preferable from the point of view ofreducing losses of optical radiation in the resonators.

[0061] Reference numeral 28 designates an output of laser 10 at whichradiation with wavelength λ_(n) may be coupled into a long-haulcommunication fiber, for example.

[0062] In the following the function of Raman laser 10 will be explainedin more detail. Raman lasers are, as already mentioned at the outset,based on an non-linear process that is referred to as spontaneous Ramanscattering. Raman scattering results from the interaction of intenselight with optical phonons in an optical medium, for example silicafibers. Raman scattering leads to a transfer of energy from one opticalbeam, for example light emitted by pump source 12 having a wavelengthλ_(p), to another optical beam, for example light of a wavelength λ₁. Ifλ₁>λ_(p) (i.e. wavelength upshift), this transfer is referred to as aStokes transition. The other case with λ₁>λ_(p) (wavelength downshift)is referred to as an Anti-Stokes transition. For achieving a Stokes oran Anti-Stokes transition in a Raman laser, it has to be ensured thatall center wavelengths λ₁, λ₂, . . . , λ_(n) of the optical reflectorsare within the Raman gain spectrum. The latter is determined by possiblevibrational modes of Raman active fiber 20.

[0063] To become more specific, pump radiation emitted by pump lightsource 12 is, during laser action, coupled into input section 14 andpropagates essentially unimpeded through input section 14 intointracavity section 18, where most of it will be converted by Ramanscattering to a radiation with longer wavelength λ₁. This radiation withupshifted wavelength is then reflected by Bragg grating 161 with centerwavelength λ₁ in the output section 16. This re-directed radiation ofwavelength λ₁ propagates back through Raman active fiber 20 where it isthen substantially converted by Raman scattering to radiation having awavelength λ₂. This radiation is now reflected by Bragg grating 142 ininput section 14. This process of wavelength conversion by Ramanscattering continues until radiation with a wavelength λ₅ is produced.This radiation is then available for utilization and may be coupled outof laser 10 via output 28.

[0064] The above discussion of the laser action is highly simplified,since typically a photon will be reflected back and forth in eachoptical cavity before it undergoes Raman scattering that results in aphoton of longer wavelength that then passes out of the cavity into thenext optical cavity.

[0065]FIG. 2 shows the energy transfer of the Raman cascade. In thisschematical representation, subsequent energy transfer from pumpradiation with wavelength λ_(p) to radiation of wavelength λ₁ and thenfrom radiation of wavelength λ_(i) to λ_(i+1) (i.e. subsequent Stokeslines) is indicated by broken arrows 301, 302, . . . , 305. As can beseen in FIG. 2, the wavelength shifts between different stages of theRaman cascade (subsequent Stokes lines) do not have necessarily to beequal. It should be noted that, due to the process of Raman scatteringand optical losses in the optical fibers, the intensity at a wavelengthλ_(i) is always smaller than the intensity at wavelength λ_(i−1). Thismeans that at output 28 of laser 10 only a fraction of the optical poweris available that has been coupled into input section 14 by pump source12.

[0066] In the laser schematically shown in FIG. 1 the center wavelengthsλ₁, λ₂, . . . , λ₅ are chosen so that

1/λ₅=1/λ₄+1/λ₃−1/λ₂.  (1)

[0067] This condition reflects energy conservation in the case offour-wave mixing. The zero-dispersion wavelength λ₀ of Raman fiber 20 ischosen so that it substantially equals (λ₃+λ₄)/2 (see FIG. 2). Thelatter condition will be referred to as the phase matching conditionthat is required for four-wave mixing to occur.

[0068] Alternatively, if the center wavelengths λ₁, λ₂, . . . , λ₅ arechosen so that

1/λ₅=2/λ₄−1/λ₃  (2)

[0069] and the zero-dispersion wavelength λ₀ of Raman fiber 20substantially equals λ₄, thus corresponding to the last but one Stokesline, the energy conservation and the phase matching condition forthree-wave mixing will be fulfilled.

[0070] The effects of four- and three-wave mixing are explained in thefollowing with reference to FIGS. 3a, 3 b, 4 a and 4 b.

[0071] Four-wave mixing is a type of optical Kerr effect and occurs whenlight of three different wavelengths is launched into a fiber, givingrise to a new wave (known as an idler), the wavelength of which does notcoincide with any of the others.

[0072]FIG. 3a shows a simplified energy level diagram in which a groundstate is designated by 40 and three excited states are designated by 42,44 and 46, respectively. In the case of four-wave mixing, a first and asecond pump photon with frequency ω₃ and ω₄, respectively, are absorbed,and a Stokes side band photon of frequency ω₅ and an anti-Stokes photonof frequency ω₂ are created.

[0073] Four-wave mixing occurs only if the following energy conservationcondition is fulfilled:

ω₅=ω₄+ω₃−ω₂  (3)

[0074] which corresponds to equation (1).

[0075] In addition the interacting photons have to obey phase matchingconditions.

[0076]FIG. 3b schematically shows the effect of four-wave mixing in thewavelength domain. As can be seen, it is possible to generate with twopump beams of wavelengths λ₃ and λ₄ radiation with upshifted wavelengthλ₅ and downshifted wavelength λ₂. For the cascaded Raman laser 10 ofFIG. 1 this effect means that an additional energy transfer takes placefrom radiation with wavelength λ₄ and radiation of wavelength λ₃ to thelaser's emitting wavelength λ₅. In FIG. 2 this effect is illustrated byadditional arrows 50 and 52, shown in dotted lines. The additionalenergy transfer results in a more direct and faster energy transfer tothe last Stokes line with emitting wavelength λ₅.

[0077]FIGS. 4a and 4 b show similar schematic diagrams as in FIGS. 3aand 3 b. The diagram of FIG. 4a differs from that of FIG. 3a withrespect to the energy levels of the excited states 62, 64 and 66. Theenergy difference between excited states 62 and the ground state 60 isthe same as the energy difference between excited state 66 and excitedstate 64. This means that two photons of identical energy (frequency ω₄)can generate two other photons, one Stokes photon with lower energy ω₅and one Anti-Stokes photon with higher energy ω₃. This case of twoidentical pump photons is referred to as degenerated four-wave mixing oralso as three-wave mixing.

[0078] The energy conservation condition in this case is

ω₅=2ω₄−ω₃  (4)

[0079] which corresponds to equation (2).

[0080]FIGS. 5 and 6 show results of experiments in which the opticalpower for different wavelengths has been measured for different opticalpump powers P_(in) for a conventional three stage cascaded Raman laserand for a three stage cascaded Raman laser according to the invention,respectively. As can be seen when comparing FIGS. 5 and 6, substantialoptical power is generated at lower wavelengths λ₁=1326 nm and λ₂=1415nm approximately for the same optical pump power P_(in). However, thelast Stokes line (emitting line of the laser) corresponding towavelength λ₃=1516 nm is not excited in the conventional Raman laser foroptical pump powers P_(in) in the range up to 5 W.

[0081] The new cascaded Raman laser exploiting three-wave mixing, to thecontrary, allows the generation of the last Stokes line with wavelengthλ₃=1516 nm with an optical pump power threshold P_(th) of only about 3W. This is due to the additional energy transfer from the second to thethird Stokes line by three-wave mixing. The new cascaded Raman laserthus allows to produce an output radiation with very low optical pumppower thresholds.

[0082]FIG. 7 shows a schematic representation of the Stokes lines in thefrequency domain of another embodiment of a laser according to theinvention. The wavelength selectors are chosen in this embodiment suchthat, in two first Stokes transitions indicated commonly by 701, a pairP1 of Stokes lines with frequencies ω₁ and ω₂ are generated from pumpradiation of frequency ω_(p). This pair P1 of Stokes lines withfrequencies ω₁ and ω₂ then undergoes another Stokes transition indicatedcommonly by 702, resulting in a second pair P2 of Stokes lines withfrequencies ω₃ and ω₄. In a further transition 703 a third pair P3 ofStokes lines with frequencies ω₅ and ω₆ is generated.

[0083] In order to achieve an energy transfer assisted by four-wavemixing between adjacent pairs P2 and P3 of Stokes lines, the followingenergy conservation conditions have to be fulfilled:

ω₆=ω₄+ω₃−ω₁  (5)

[0084] and

ω₅=ω₄+ω₃−ω₂  (6)

[0085] or, if written in the wavelength domain,

1λ₆=1/λ₄+1/λ₃−1/λ₁  (7)

[0086] and

1λ₅=1/λ₄+1/λ₃−1/λ₂,  (8)

[0087] respectively. The phase matching condition will be

λ₀=(λ₃+λ₄)/2.  (9)

[0088] The frequency differences of equations (5) and (6) are indicatedin FIG. 7 by horizontal arrows.

[0089] If these conditions are fulfilled, the energy transfer to thelast two Stokes lines with frequencies ω₅ and ω₆ is assisted byfour-wave mixing. In effect, two lasers as shown in FIG. 1 are thuscombined in a single device, resulting in a cascaded Raman laser thatmay, for example, be configured such that it has not only one but twolow-power outputs.

[0090]FIG. 8 shows a schematic representation of the Stokes lines in thefrequency domain of still another embodiment of a laser according to theinvention. This embodiment differs from that shown in FIG. 7 in thatthree-wave mixing instead of four-wave mixing assists the Stokestransitions.

[0091] Again, a pair P1 of Stokes lines with frequencies ω₁ and ω₂ aregenerated from pump radiation of wavelength ω_(p) in two first Stokestransitions indicated commonly by 801. This pair P1′ of Stokes lineswith frequencies ω₁ and ω₂ then undergoes another Stokes transitionindicated by 802, resulting in a single Stokes line with frequency ω₃.In a further transition 803 a second pair P2′ of Stokes lines withfrequencies ω₄ and ω₅ are generated.

[0092] In order to achieve an energy transfer assisted by three-wavemixing the following energy conservation conditions have to befulfilled:

ω₅=2ω₃−ω₁  (10)

[0093] and

ω₄=2ω₃−ω₂  (11)

[0094] or, if written in the wavelength domain,

1/λ₅=2/λ₃+1/λ₁  (12)

[0095] and

1/λ₄=2/λ₃−1/λ₂  (13)

[0096] respectively. The phase matching condition will be

λ₀=λ₃.   (14)

[0097] Again the result is a cascaded Raman laser that allows to coupleout two stable low-power outputs.

1. A cascaded Raman laser comprising: a) a pump radiation sourceemitting at a pump wavelength λ_(p), b) an input section and an outputsection made of an optical medium, each section comprising wavelengthselectors for wavelengths λ₁, λ₂, . . . , λ_(n−k), where n≧3,λ_(p)<λ₁<λ₂< . . . <λ_(n−1)<λ_(n) and λ_(n−k+1), λ_(n−k+2), . . . ,λ_(n) being k≧1 emitting wavelengths of the laser, and c) an intracavitysection that is made of a non-linear optical medium, has azero-dispersion wavelength λ₀ and is disposed between the input and theoutput section, wherein d) the wavelengths λ₁, λ₂, . . . , λ_(n−k) ofthe wavelength selectors and the zero-dispersion wavelength λ₀ of theintracavity section are chosen such that energy is transferred betweenradiation of different wavelengths by multi-wave mixing.
 2. The laser ofclaim 1, wherein at least one of the emitting wavelengths λ_(n−k+1),λ_(n−k+2), . . . , of the laser is involved in multi-wave mixing.
 3. Thelaser of claim 1, wherein the wavelengths λ₁, λ₂, . . . , λ_(n−k) of thewavelengths selectors are chosen so that energy transfer by multi-wavemixing involves at least three adjacent wavelengths.
 4. The laser ofclaim 3, wherein the wavelengths λ₁, λ₂, . . . , λ_(n−k) of thewavelength selectors are chosen so that1/λ_(i)=1/λ_(i−1)+1/λ_(i−2)−1/λ_(i−3), where i=3, 4, . . . , n, and thatthe zero-dispersion wavelength λ₀ of the intracavity sectionsubstantially equals (λ_(i−1)+λ_(i−2))/2.
 5. The laser of claim 3,wherein the wavelengths λ₁, λ₂, . . . , λ_(n−k) of the wavelengthsselectors are chosen so that 1/λ_(i)=2/λ_(i−1)−1/λ_(i−2), where i=3, 4,. . . , n, and that the zero-dispersion wavelength λ₀ of the intracavitysection substantially equals λ_(i−1).
 6. The laser of claim 1, whereinthe wavelengths λ₁, λ₂, . . . , λ_(n−k) of the wavelengths selectors arechosen so that energy transfer by multi-wave mixing involves at leastthree nonadjacent wavelengths.
 7. The laser of claim 6, wherein k=2 andthat the wavelengths λ₁, λ₂, . . . , λ_(n−2) of the wavelengthsselectors are chosen so that 1/λ_(i)=1/λ_(i−2)+1/λ_(i−3)−1/λ_(i−5) and1/λ_(i−1)=1/λ_(i−2)+1/λ_(i−3)−1/λ_(i−4), where i=5, 6, . . . , n, andthat the zero-dispersion wavelength λ₀ of the intracavity sectionsubstantially equals (λ_(i−2)+λ_(i−3))/2.
 8. The laser of claim 6,wherein k=2 and that the wavelengths λ₁, λ₂, . . . , λ_(n−1) of thewavelengths selectors are chosen so that 1λ_(i)=2/λ_(i−2)−1/λ_(i−4) and1/λ_(i−1)=2/λ_(i−2)−1/λ_(i−3), where i=5, 6, . . . , n, and that thezero-dispersion wavelength λ₀ of the intracavity section substantiallyequals λ_(i−2).
 9. The laser of claim 1, wherein for each emittingwavelength an additional wavelength selector for wavelength λ_(n−k+1),λ_(n−k+2), . . . , λ_(n), respectively, is provided in the input section(14) and in the output section.
 10. The laser of claim 1, wherein thewavelength selectors are reflectors having center wavelengths λ₁, λ₂, .. . , λ_(n−k). US-Fassung.doc
 22. 10.2002